Portable equipment for luminescence lifetime measurements on surfaces

Luminescence Lifetime-Based Sensing Platform Based on Cyclometalated Iridium(III) Complexes for the Detection of Perfluorooctanoic Acid in Aqueous Samples

Luminescence lifetimes are an attractive analytical method for detection due to its high sensitivity and stability. Iridium probes exhibit luminescence with long excited-state lifetimes, which are sensitive to the local environment. Perfluorooctanoic acid (PFOA) is listed as a chemical of high concern regarding its toxicity and is classified as a "forever chemical". In addition to strict limits on the presence of PFOA in drinking water, environmental contamination from industrial effluent or chemical spills requires rapid, simple, accurate, and cost-effective analysis in order to aid containment. Herein, we report the fabrication and function of a novel and facile luminescence sensor for PFOA based on iridium modified on gold surfaces. These surfaces were modified with lipophilic iridium complexes bearing alkyl chains, namely, IrC6 and IrC12, and Zonyl-FSA surfactant. Upon addition of PFOA, the modified surfaces IrC6-FSA@Au and IrC12-FSA @Au show the largest change in the red luminescence signal with changes in the luminescence lifetime that allow monitoring of PFOA concentrations in aqueous solutions. The platform was tested for the measurement of PFOA in aqueous samples spiked with known concentrations of PFOA and demonstrated the capacity to determine PFOA at concentrations >100 μg/L (240 nM).

Visualising varnish removal for conservation of paintings by fluorescence lifetime imaging (FLIM)

The removal of varnish from the surface is a key step in painting conservation. Varnish removal is traditionally monitored by examining the painting surface under ultraviolet illumination. We show here that by imaging the fluorescence lifetime instead, much better contrast, sensitivity, and specificity can be achieved. For this purpose, we developed a lightweight (4.8 kg) portable instrument for macroscopic fluorescence lifetime imaging (FLIM). It is based on a time-correlated single-photon avalanche diode (SPAD) camera to acquire the FLIM images and a pulsed 440 nm diode laser to excite the varnish fluorescence. A historical model painting was examined to demonstrate the capabilities of the system. We found that the FLIM images provided information on the distribution of the varnish on the painting surface with greater sensitivity, specificity, and contrast compared to the traditional ultraviolet illumination photography. The distribution of the varnish and other painting materials was assessed using FLIM during and after varnish removal with different solvent application methods. Monitoring of the varnish removal process between successive solvent applications by a swab revealed an evolving image contrast as a function of the cleaning progress. FLIM of dammar and mastic resin varnishes identified characteristic changes to their fluorescence lifetimes depending on their ageing conditions. Thus, FLIM has a potential to become a powerful and versatile tool to visualise varnish removal from paintings.

Graphical Abstract

Multi-Technique Diagnostic Analysis of Plasters and Mortars from the Church of the Annunciation (Tortorici, Sicily)

Plasters and mortars of the Church of the Annunciation (Tortorici, Sicily) were characterized, for the first time, both at the elemental and molecular levels, by means of portable X-ray fluorescence (XRF) and Raman spectroscopy, to achieve information on the “state of health” of the whole structure. The understanding of their degradation mechanisms and the identification of consequent degradation patterns can define the environmental factors responsible for interpreting the potential pathological forms that can impact the general building vulnerability. In this sense, the results obtained in this article provide relevant information to identify and address both the characterization of building materials and the fundamental causes of their deterioration. At the same time, if coupled with the attempt to supply a chronological order of the major restoration interventions carried out on the investigated site, they provide new insights to calibrate the models for building vulnerability studies.

Effect of hydrogen bonding interaction on the photophysics of α-amino-orcein

This paper reports for the first time a detailed spectroscopic investigation into the ground- and excited-state properties of α-amino-orcein (α-AO), one of the main components of the orcein dye, in solvents of different proticity and water at different pHs. In order to gain insight into the nature of the involved transitions and excited state deactivation pathways, the study was carried out by means of UV-Visible steady state and ultrafast spectroscopic techniques with the support of quantum mechanical calculations (DFT and TDDFT). The results highlight that the photophysical and photodynamic behaviour of α-AO are highly sensitive to the solvent proticity and pH. In particular, protic environment induces a red shift (55 nm) of the absorption spectrum together with a relevant decrease of the fluorescence quantum yield (from 0.19 in acetonitrile to 6.6 × 10^-3 in methanol) and radiative rate constant (two orders of magnitude). A notable red shift is also caused by increasing the pH leading the molecule from monocationic to neutral and then monoanionic form through two deprotonation steps (pKa = 3.539 ± 0.006 and 11.180 ± 0.006). Following deprotonation, the molecule assumes spectral and photophysical properties very similar to those retrieved in protic media. The observed behaviour has been rationalized through the occurrence of hydrogen bonding, likely involving to a greater extent the carbonyl oxygen of α-AO and the protic solvent, that favours the charge delocalization on the whole chromophore as well as fast non-radiative excited state deactivation. The ultrafast spectroscopic investigation revealed in fact the presence, in protic solvent, of a short living component (tens of picoseconds), assignable to solvent complexed S₁ state, alongside the long living component (few nanoseconds) observed in aprotic media and attributed to the solvent free S₁ state. The results achieved in this study for α-AO provides an important contribution to the interpretation of absorption and fluorescence features of orcein dye mixture in more complex systems (protein based substrates within the many aspects of the cultural heritage and biomedical field) where hydrogen bonds are expected to play a crucial role in mediating the interaction with the environment.

Analytical investigations on the Coronation Gospels manuscript

The Coronation Gospels or Krönungsevangeliar is a manuscript kept in Vienna at the Kunsthistorisches Museum Wien, datable to the end of VIII century A.D. and produced at Charlemagne court. It is an example of a purple codex, i.e. its parchment is coloured in purple. It has to be considered as one of the most important medieval codices, according to its use to take oath in the coronation ceremony of kings and emperors of the Holy Roman Empire up to 1792. In order to gather information of the manufacture of the manuscript and its present conservation state, a diagnostic investigation campaign has been carried out in situ with totally non-invasive techniques. X-ray Fluorescence Spectrometry (XRF), UV-visible diffuse reflectance spectrophotometry with optical fibres (FORS), spectrofluorimetry, optical microscopy and multispectral analysis have been applied in order to identify the colourants used in the decoration of the manuscript, with the main concern to the dye used to impart the purple hue to the parchment. The information collected was useful in order to address some of the questions raised by art historians concerning its history.

On the identification of folium and orchil on illuminated manuscripts

The identification of the two purple dyes folium and orchil has rarely been reported in the analysis of painted artworks, especially when analysing illuminated manuscripts. This is not consistent with the fact that ancient literary sources suggested their use as substitutes for the more expensive Tyrian purple dye. By employing non-invasive spectroscopic techniques, the present work demonstrates that these dyes were actually widely used in the production of ancient manuscripts. By employing UV-visible diffuse reflectance spectrophotometry with optic fibres (FORS) and spectrofluorimetry, the abundant identification of both dyes on medieval manuscripts was performed by comparing the spectra recorded on ancient codices with those obtained on accurate replicas of dyed or painted parchment. Moreover, examples are also reported whereby the considered purple dyes were used in mixtures with other colourants. The overall information obtained here allowed us to define new boundaries for the time range in which orchil and folium dyes were used which is wider than previously thought, and to focus on their particular uses in the decoration of books.

Mobile Spectroscopic Instrumentation in Archaeometry Research

Mobile instrumentation is of growing importance to archaeometry research. Equipment is utilized in the field or at museums, thus avoiding transportation or risk of damage to valuable artifacts. Many spectroscopic techniques are nondestructive and micro-destructive in nature, which preserves the cultural heritage objects themselves. This review includes over 160 references pertaining to the use of mobile spectroscopy for archaeometry. Following a discussion of terminology related to mobile instrumental methods, results of a literature survey on their applications for cultural heritage objects is presented. Sections devoted to specific techniques are then provided: Raman spectroscopy, X-ray fluorescence spectrometry, Fourier transform infrared spectroscopy, laser-induced breakdown spectroscopy, and less frequently used techniques. The review closes with a discussion of combined instrumental approaches.

Twisting in the excited state of an N-methylpyridinium fluorescent dye modulated by nano-heterogeneous micellar systems

The fluorescence of an N-methylpyridinium dye was modulated by nano-heterogeneous micellar systems, where its excited state twisting is gradually impaired by the increasing viscosity of the surrounding environment.

Doxycycline and oxytetracycline loading of a zwitterionic amphoteric surfactant-gel and their controlled release

Oxytetracycline (OX) and doxycycline (DX) are antibiotics belonging to the family of tetracyclines. We present a UV-Visible steady state and time-resolved experimental study of OX and DX and their biologically active Mg(2+) complexes loaded within a hydrogel matrix. Hydrogels are a three dimensional network of worm-like micelles, mutually intertwined, forming a pattern of hydrophobic domains and water pools. We resorted to a hydrogel, made of a zwitterionic N-oxide surfactant (p-dodecyloxybenzyldimethylamine N-oxide, pDoAO), which showed promising features as a drug vehicle. The spectral and photophysical properties of the drugs are significantly altered by the inclusion in the hydrophobic domains of the gel and these variations are indicators of the permeation ratio of the drug in between the micelles forming the gel network. We thus get a clear picture of the distribution of the drug molecules and metal chelates into the two different kinds of environment, where the hydrophobic domains are also able to cause a gel-induced deprotonation of these two drugs. Furthermore, the amphoteric nature of the surfactant is responsible for its peculiar acid-base behaviour: under acidic pH conditions, the surfactant gets protonated and the stability of the gel network is damaged. This feature can be thus exploited for the pH controlled release of the tetracycline drugs.

A diagnostic study on folium and orchil dyes with non-invasive and micro-destructive methods

Folium and orchil are dyes of vegetal origin. Folium is obtained from Chrozophora tinctoria (L.) A. Juss., whereas orchil is obtained from Roccella and other genera of lichens. These dyes were used in the past to impart purple hue to paintings and textiles as substitutes for the more prised Tyrian purple dye, obtained from shellfish. Despite several citations in ancient technical treatises dating back at least to the Greek-Roman age, the identification of these dyes in artworks is rare. In the case of folium, an additional drawback is that its composition is presently unknown. In this work different non-invasive (FT-IR, FT-Raman, fibre optic reflectance spectrophotometry, spectrofluorimetry, X-ray fluorescence spectrometry) and micro-invasive (surface enhanced Raman spectroscopy, matrix assisted laser desorption ionisation-time of flight-mass spectrometry, inductively coupled plasma-mass spectrometry) techniques were used in order to increase the diagnostic information available on these dyes. Measurements were carried out on the dyes extracted from raw materials and on painted or dyed parchments. The possibility to distinguish between folium and orchil by chemical analysis is discussed.

Time-resolved photoluminescence spectroscopy and imaging: new approaches to the analysis of cultural heritage and its degradation

Applications of time-resolved photoluminescence spectroscopy (TRPL) and fluorescence lifetime imaging (FLIM) to the analysis of cultural heritage are presented. Examples range from historic wall paintings and stone sculptures to 20th century iconic design objects. A detailed description of the instrumentation developed and employed for analysis in the laboratory or in situ is given. Both instruments rely on a pulsed laser source coupled to a gated detection system, but differ in the type of information they provide. Applications of FLIM to the analysis of model samples and for the in-situ monitoring of works of art range from the analysis of organic materials and pigments in wall paintings, the detection of trace organic substances on stone sculptures, to the mapping of luminescence in late 19th century paintings. TRPL and FLIM are employed as sensors for the detection of the degradation of design objects made in plastic. Applications and avenues for future research are suggested.

Non-invasive investigation on a VI century purple codex from Brescia, Italy

Purple codices are among the most relevant and prestigious book productions of Late Antique and Medieval age. They usually contained texts from Holy Writings written with golden or silver inks on parchment dyed in a purple hue. According to the tradition, the colour of parchment was obtained by the well renowned Tyrian purple dye. From the material point of view, however, very little is known about the compounds actually used in the manufacture of these manuscripts. Presently, the information available is limited to the ancient art treatises, with very few diagnostic evidences supporting them and, moreover, none confirming the presence of Tyrian purple. It is more than apparent, then, the need to have at disposal larger and more complete information at the concern, in order to verify what came to us from the literary tradition only. In this study, preliminary results are presented from non-invasive investigation on a VI century purple codex, the so-called CodexBrixianus, held in the Biblioteca Civica Queriniana at Brescia (Italy). Analyses were carried out with XRF spectrometry, UV-visible diffuse reflectance spectrophotometry, molecular spectrofluorimetry and optical microscopy. The results suggest the hypothesis that Tyrian purple had been used as a minor component mixed with other less precious dyes such as folium or orchil.

The Book of Kells: a non-invasive MOLAB investigation by complementary spectroscopic techniques

This paper highlights the efficacy of non-invasive portable spectroscopy for assessing the execution technique and constituent materials in one of the most important medieval manuscripts, the Book of Kells. An aimed campaign of in situ measurements by the MObile LABoratory (MOLAB) has analyzed its elemental composition and vibrational and electronic molecular properties. The ample analytical toolbox has afforded complementary diagnostic information of the pigment palette permitting the characterization of both inorganic and organic materials as pigments and dyes in the white, purple, blue, red, orange, green and black areas. In particular, the novel widespread use of calcinated gypsum (anhydrite) as both a white pigment and in correlation to the organic dyes in this manuscript has been noted. The non-invasive identification of the organic dye orchil is significant considering its rare non invasive detection in medieval manuscripts. Finally the occurrence of particular alterations of the organic black areas giving rise to calcium carboxylate and calcium oxalate has been specifically highlighted. Importantly, this work elaborates complex aspects of the employed painting materials which have given rise to numerous significant points of interest for a more elaborate understanding of this Irish treasure.

Non invasive analysis of miniature paintings: proposal for an analytical protocol

The characterisation of palettes used in manuscript illumination is a hard analytical task, due to value and fragility of the analysed items. Analysis on miniatures must be necessarily non-invasive and fast and requires the use of several techniques since no single technique is able to provide all information needed. In this work a four-step analytical protocol is proposed for non-invasive in situ characterisation of miniature paintings. The protocol allows the identification of coloured materials through the use in sequence of complementary techniques, so as to fully exploit the information given by each instrument. Preliminarily to the instrumental investigations on ancient books and miniatures is the compilation of spectroscopic databases obtained from "standard" samples prepared on parchment, according to recipes described in medieval artistic treatises. The protocol starts with an extensive investigation with UV-visible spectrophotometry in reflectance mode, collecting spectra from all the most significant painted areas in the manuscript; chemometric classification is then performed on the spectra to highlight areas possibly containing the same materials. The second step involves in-depth inspection of miniatures under optical microscopy that guides the interpretation of reflectance spectra. XRF spectrometry is then performed to characterise pigments and metal layers, to verify the presence of overlapping layers, to identify mordants in lakes and to recognise minor components that may yield information concerning provenance; in addition, chemometric classification can be performed on element concentrations to highlight similar areas. Finally, Raman spectroscopy is used to shed light on the uncertain cases, if still present. Such a procedure offers a wealth of information without causing stress to the manuscripts under analysis.

Ultraviolet-visible absorption and luminescence properties of quinacridone-barium sulfate solid mixtures

The absorption and emission spectral features of the dye quinacridone (QA) have been studied in solution and in the solid phase. In the solid phase, QA has been investigated as pure microcrystalline powder and mixed with barium sulfate (BaSO(4)) in different mass percentages. Two kinds of QA-BaSO(4) mixtures have been prepared: physical blends of the two microcrystalline powders, and mixtures with gum arabic as binder. The latter was used in painting mock-ups. Luminescence properties of the mixtures have been investigated using both steady-state and time-resolved techniques. The prepared samples have allowed the Kubelka-Munk correction model, formulated for the emission spectra, to be tested. Moreover, the luminescence decay profiles have been analyzed using the maximum entropy method (MEM) and the nonlinear least-squares method. All the results obtained highlight how physical (self-absorption) and chemical (composition of the microenvironment) factors can influence the spectral and kinetic properties of dyes. These factors should always be taken into account in the diagnostic activity applied to works of art.

Fluorescence spectroscopy: a powerful technique for the noninvasive characterization of artwork

After electronic excitation by ultraviolet or visible radiation, atoms and molecules can undergo thermal or radiative deactivation processes before relaxing to the ground state. They can emit photons with longer wavelengths than the incoming exciting radiation, that is, they can fluoresce in the UV-vis-near-infrared (NIR) range. The study of fluorescence relaxation processes is one of the experimental bases on which modern theories of atomic and molecular structure are founded. Over the past few decades, technological improvements in both optics and electronics have greatly expanded fluorimetric applications, particularly in analytical fields, because of the high sensitivity and specificity afforded by the methods. Using fluorimetry in the study and conservation of cultural heritage is a recent innovation. In this Account, we briefly summarize the use of fluorescence-based techniques in examining the constituent materials of a work of art in a noninvasive manner. Many chemical components in artwork, especially those of an organic nature, are fluorescent materials, which can be reliably used for both diagnostic and conservative purposes. We begin by examining fluorimetry in the laboratory setting, considering the organic dyes and inorganic pigments that are commonly studied. For a number of reasons, works of art often cannot be moved into laboratories, so we continue with a discussion of portable instruments and a variety of successful "field applications" of fluorimetry to works of cultural heritage. These examples include studies of mural paintings, canvas paintings, tapestries, and parchments. We conclude by examining recent advances in treating the data that are generated in fluorescence studies. These new perspectives are focused on the spectral shape and lifetime of the emitted radiation. Recent developments have provided the opportunity to use various spectroscopic techniques on an increasing number of objects, as well as the ability to fully characterize very small amounts of sample, either in a laboratory setting or on site. Thus, a new technological highway is open to scientists; it is still difficult to navigate but offers an enormous potential for investigating objects without touching them. Fluorescence spectroscopy is one of the most important of these techniques.

Bright light: microspectrofluorimetry for the characterization of lake pigments and dyes in works of art

Color is an important component in the perception of beauty and in an artist's original intent when creating a work. Better conservation of our cultural heritage requires detailed knowledge of artwork materials and the complex evolution they have endured over time. Organic dyes have been used from ancient times, and their characterization is a challenge that has been successfully addressed over the past few years by the development of advanced techniques, such as microspectrofluorimetry. In this Account, we describe the application of microspectrofluorimetry to the study of medieval illuminations, paint cross sections, millenary textiles, and wall paintings. In our research into color in medieval Portuguese illuminations, we chose to emphasize the importance of the experimental design and the use of microspectrofluorimetry in the context of other analytical techniques, such as microFTIR, microRaman, and micro-X-ray fluorescence (microXRF). Within this framework, we were able to unveil the full complexity of a medieval colorant and to address issues not yet explored, such as the influence of Arab, Jewish, and Christian cultures on the production and underlying technology of Portuguese illuminations. The analysis of individual pigment particles or aggregates (by excitation with an 8 mum diameter spot) in paint cross sections from works by Vincent van Gogh and Lucien Pissarro highlights the technique's advantage of high spatial resolution. Its high spectral resolution proved to be useful not only for better characterizing the dyes used to color Andean textiles but also for detecting mixtures of relevant chromophores; the emission signals for the reds in Paracas and Nasca textiles were shown to be due to the presence of purpurin and pseudopurpurin. Finally, the complexity of the study of yellow dyes and the importance of accurate historical reproductions is addressed in a study of Asian organic colorants on historic Chinese wall paintings. Microspectrofluorimetry offers high sensitivity, selectivity, fast data acquisition, good spatial resolution, and the possibility of in-depth profiling. It has proved to be an invaluable analytical tool in identifying dyes and lake pigments in works of art. As Saint-Exupéry's protagonist said in Le Petit Prince, "L'essentiel est invisible pour les yeux," or "What is essential is invisible to the eye"--but it may be unveiled with kind love, a prepared mind, and a little help from microspectrofluorimetry.

In situ noninvasive study of artworks: the MOLAB multitechnique approach

Driven by the need to study precious and irreplaceable artworks without compromising their integrity, researchers have undertaken numerous efforts to develop noninvasive analytical tools and methodologies that can provide a chemical description of cultural heritage materials without any contact with the object. The challenge is that artworks are made of complex mixtures, often with heterogeneous and unknown layered materials. Their components must be identified over a range of size scales, from the molecular identification of constituent compounds to the mapping of alteration phases. In this Account, we review recent research in spectroscopic techniques accessible from the mobile laboratory (MOLAB). The lab is equipped with an array of state-of-the-art, portable, and noninvasive instruments specifically tailored to tackle the different issues confronted by archaeologists, curators, and conservators. The MOLAB approach is suitable for studying a variety of objects, from ceramics to manuscripts or from historical wall paintings to contemporary canvases. We begin by discussing issues related to the acquisition and interpretation of reflectance or backscattered spectra from the surface of heterogeneous materials. Then we show how the selectivity needed for the noninvasive identification of pigments in paintings, even in mixtures or in layered matrices, can be acquired by combining elemental information from X-ray fluorescence with molecular and structural insights from electronic and vibrational spectroscopies. Discriminating between original pigments and restoration retouches is possible, even when both comprise similar chromophores, as highlighted in the study of paintings by Jordaens and Raphael. The noninvasive approach permits the examination of a very large number of artworks with a virtually limitless number of measurements. Thus, unexpected and uncommon features may be uncovered, as in the case of a lead pyroantimonate yellow doped with zinc that was discovered by micro-Raman and X-ray fluorescence on an Italian Renaissance majolica. For characterizing binding media, we discuss the strengths and limitations of using mid- and near-FTIR (Fourier transform infrared) spectroscopies supported by a multivariate statistical analysis, detailing the study of organic materials in a wall painting by Perugino and a survey of the painting technique on 18 contemporary paintings by Burri. In Michelangelo's David, we show how the noninvasive mapping of contaminants and alteration phases might inform decisions on preventive conservation plans. The multitechnique MOLAB approach overcomes the intrinsic limitation of individual spectroscopic methods. Moreover, the ability to analyze artworks without the need to move them is an invaluable asset in the study and preservation of cultural heritage.

Immunodetection of proteins in ancient paint media

Diagnostic immunology is a powerful tool, widely used in clinical and biochemical laboratories for detecting molecules. In recent years, the technique has been adapted to materials sciences as a result of the extensive advances achieved in immunology. Today, many companies supply custom antibodies as well as new high-performance bioprobes for virtually any use. The idea of using immunodetection in the field of conservation science is not new. This analytical methodology is, in fact, particularly attractive for investigating biopolymers in painting materials; it is highly sensitive and selective with respect to the biological source of the target molecules. Among biopolymers, proteins have been widely used in the past as painting binders, adhesives, and additives in coating layers. An accurate assessment of these materials is necessary to obtain deeper insights into an artist's technique as well as to design proper restoration and conservation methods. In spite of the diagnostic potential offered by immunodetection-based techniques, some analytical drawbacks had, until recently, limited their use in routine applications in conservation science. In this Account, we highlight the most important results achieved in our research on the development of analytical methodologies based on the use of enzyme-linked immunosorbent assay (ELISA) and immuno-fluorescence microscopy (IFM) techniques for the highly sensitive and specific identification of proteins in artistic and archeological materials. ELISA and IFM offer two alternative analytical routes to this final goal: ELISA provides a fast, cost-effective, quantitative analysis of microsamples put in solution, whereas IFM combines the immunodetection of the targeted molecules with the characterization of their spatial distribution. The latter approach is of great value in the stratigraphic investigation of paintings. We discuss the limits and strengths of these methodologies in the context of the complex matrixes usually found in the investigated materials and the prolonged aging that they have undergone. Immunology is a relatively new technique in conservation science, providing a rich new field for innovation. We see two areas that are particularly ripe for future contributions. The commercial manufacture of antibodies specifically tailored for use in cultural heritage studies holds enormous potential. Moreover, the need for further refinement of detection systems in immuno-fluorescence techniques, especially the suppression of the autofluorescence background in painting materials, offers an abundance of opportunities for researchers. Immunology is a relatively new technique in conservation science, providing a rich new field for innovation.

Complexation of apigenin and luteolin in weld lake: a DFT/TDDFT investigation

A DFT-TDDFT investigation on the aluminium complexation of apigenin and luteolin has been carried out. We have focused our attention on these hydroxyflavonoids, which are the main components of weld, one of the earliest natural dyestuff used in art. In particular, weld, upon complexation with Al(iii) forms a highly prized lake which has been widely used in medieval manuscripts and easel paintings for its rich yellow colour and transparency. The experimental spectra of apigenin and luteolin upon addition of increasing [Al(3+)] show a general red-shift of the lowest absorption bands of both flavonoids spectra, associated with the presence of two and three isosbestic points for apigenin and luteolin, respectively. The molecular geometries of all the Al-apigenin and -luteolin complexes have been optimized, followed by calculation of the formation Gibbs free energies and UV-vis absorption spectra. The comparison between the computed absorption spectra of the Al-flavonoid complexes and the experimental ones corresponding to various limit [Al(3+)] concentrations has been used to discriminate between the possible complexation modes as well as the stoichiometry ratio. We have thus been able to associate specific Al-apigenin (-luteolin) complexes with the experimental absorption spectra as a function of the [Al(3+)] concentration, thus providing insights into the aluminium complexation of these hydroxyflavonoids and most importantly into the weld lake composition.

Application of the Kubelka-Munk correction for self-absorption of fluorescence emission in carmine lake paint layers

The variations of the fluorescence emission of carmine lake travelling through an absorbing and scattering medium, such as a paint layer, were investigated by ultraviolet (UV)-visible absorption, fluorescence spectroscopy, and imaging techniques. Samples of the lake were studied in dilute and saturated solutions, on a reference test panel and a real case study. Relevant spectral modifications have been observed as a function of the lake concentration mainly consisting of a fluorescence quenching, red shift of emission maxima, and deformation of emission band. The application of a correction factor based on the Kubelka-Munk model allowed fluorescence spectra obtained in solution and on painted samples of known composition to be compared and correlated, highlighting that the fluorescence of the lake within paint layers is affected by both self-absorption and aggregation phenomena. This approach has been successfully applied on a painting by G. Vasari for the noninvasive identification of carmine lake. The results reported here emphasize the necessity of taking physical phenomena into account in the interpretation of the fluorescence spectra for a proper and reliable characterization and identification of painting materials in works of art.

The exceptional near-infrared luminescence properties of cuprorivaite (Egyptian blue)

Cuprorivaite (CaCuSi(4)O(10), also known as Egyptian blue) exhibits an exceptionally high emission quantum efficiency in the near-infrared region (lambda(max) = 910 nm, Phi(EM) = 10.5%) and a long excited state lifetime (107 mus); these properties make it appealing for several applications in the fields of biomedical analysis, telecommunications and lasers.

In-situ fluorimetry: a powerful non-invasive diagnostic technique for natural dyes used in artefacts. Part II. Identification of orcein and indigo in Renaissance tapestries

In this paper, three Renaissance tapestries depicting scenes painted by Raffaello Sanzio, conserved at the Vatican Museum, were investigated using in-situ UV-Visible fluorimetric measurements. The results show that this technique is suitable for the detection of natural organic colorants used for dyeing the threads woven in these tapestries. The emission signals detected on red-purple colours were assigned to the colorant orcein and those on different nuances of blue and green colours to indigo by comparison with data from reference laboratory samples. The assignments were supported by chromatographic experiments carried out on threads taken from the back side of the tapestry in the same points analysed by spectrofluorimentry.